It depends on how you will be testing if the user is an Admin or User.
The standard way to do this is to define groups, and then add users into the groups. Your application then tests to see if the user is a member of a group, and allow\deny access to resources accordingly.
If you are using Windows authentication with Active Directory, you can define your AD groups and just use WindowsPrincipal objects to test for group member ship.
WindowsPrincipal wp = new WindowsPrincipal(WindowsIdentity.GetCurrent());
bool result = wp.IsInRole(@"domain\Role");
You can also use any other database that has the ability to store group \ user information and create your own principal objects
I wrote an article on using AspNetDb (usually used for Asp Net membership information) with WinForms
WinForms - Using a custom principal with AspNetDb